The situation is this:
I have set message size restrictions in my main config file using @message_size_limit_maps (for example: [email protected] => 3*1024*1024)

In ISPC the [email protected] mailbox spam policy is set to 'Normal' which has it's message size restriction set to zero. Amavisd interprets this value as disable which overrides my main config file value. Manually setting the value to NULL in the spamfilter_policy table fixes the situation and amavisd adheres to the value set in the main config file.

Is this behavior intended?

It's problematic for me as I would like to enforce size limits that a user cannot override by setting or changing their policy. Chaning the default value of zero to NULL would fix this.